This application claims the benefit of a Japanese Patent Application No.2000-107077 filed Apr. 7, 2000, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to magnetic recording mediums and magnetic storage apparatuses, and more particularly to a magnetic recording medium and a magnetic storage apparatus which are suited for high-density recording.
2. Description of the Related Art
The recording density of longitudinal magnetic recording mediums, such as magnetic disks, has been increasing considerably, due to the reduction of media noise and the development of magnetoresistive and high-sensitivity spin-valve heads. Recording densities above 50 Gb/in2 have recently been demonstrated for hard disks. The demand for greater recording densities for better performing computers is however showing an increasing trend imposing greater challenges for the recording mediums and other component design.
FIG. 1 is a cross sectional view showing an important part of a typical longitudinal magnetic recording medium. The magnetic recording medium is comprised of a substrate 1, a Cr or Cr-based underlayer 2, a Co-based magnetic layer 3 where information is written, and a C or DLC overlayer 4 which are stacked as shown in FIG. 1. An organic lubricant is coated on the overlayer 4.
Lowering the media noise involves writing sharper magnetic transitions in the magnetic layer 3. This is generally achieved by increasing the media coercivity, decreasing the thickness of the magnetic layer 3, decreasing the grain size and grain size distribution of the magnetic layer 3, and magnetically isolating the grains of the magnetic layer 3.
A higher signal-to-noise ratio (hereinafter simply referred to as SNR) is obtained when the grain sizes of the magnetic layer 3 are small and the distribution of the grain sizes is narrow. One approach to achieve small grain sizes for the magnetic layer 3 is to reduce the grain diameters of the underlayer 2. Using a Cr-based alloy including Mo, V, W, Ti or the like will lead to smaller grain diameters of the underlayer 2. In addition, a bi-layer underlayer structures sometimes lead to smaller grain diameters than single-layer underlayer structures. Addition of B to the Co-based alloy of the magnetic layer 3 also reduces the grain sizes of the magnetic layer 3.
However, the small grains of the magnetic layer 3 adversely affect the thermal stability of the magnetic recording medium. Normally, the thermal stability of the magnetic layer 3 is represented by how large a thermal stabilization factor KuV/kT is, where Ku denotes the magnetic anisotropy, V the volume of the grain, T the temperature, and k the Boltzmann constant. In order to obtain thermally stable small grains, the magnetic anisotropy Ku has to be increased.
The magnetic anisotropy field Hk is defined as Hk=2Ku/Ms, where Ms denotes the saturation magnetization. A large magnetic anisotropy field Hk means a large coercivity Hc at the nanosecond regime where normally, the writing of the information occurs for a high-density recording mediums with high data transfer rates. High coercivity Hc at writing frequencies puts severe limitations on the write heads, since a large write current is required in order to write information on such magnetic recording mediums. Write currents, which can be produced by write heads, are severely limited due to difficulties in developing write heads with high magnetic moment.
The overwrite performance, which is the ability to write new data over previously written data, deteriorates for magnetic recording mediums with high magnetic anisotropy field Hk, as shown in FIG. 2. FIG. 2 is a diagram showing the overwrite performance of magnetic recording mediums having various coercivities at 100 ns sweeping time. In FIG. 2, the ordinate indicates the overwrite performance OW (dB), and the abscissa indicates the coercivity Hc (Oe) at 100 ns sweeping time. As the magnetic anisotropy Ku increases to thereby increase the magnetic anisotropy field Hk, the overwrite performance becomes restricted as may be seen from FIG. 2. But such an increase in the magnetic anisotropy field Hk with increasing magnetic anisotropy Ku can be restricted or reversed, if the increasing magnetic anisotropy Ku is also accompanied by an increase in the saturation magnetization Ms.
The magnetic anisotropy Ku of the magnetic layer 3 is normally increased by adding elements such as Pt to the Co-based alloy which forms the magnetic layer 3. However, such an increase in the magnetic anisotropy Ku by the addition of Pt is inevitably accompanied by a decrease in the saturation magnetization Ms which over a range restricts the overwrite performance. Alternatively, the Co-content of the magnetic layer 3 may be increased in order to increase the magnetic anisotropy Ku. Increasing the Co-content of the magnetic layer 3 not only increases the magnetic anisotropy Ku, but also increases the saturation magnetization Ms.
The magnetic layer 3 is usually made of a CoCr alloy in which Cr helps segregating the Co grains from each other. Such segregation is very important in achieving low media noise. When producing the commonly used magnetic recording mediums having the magnetic layer 3 which is made of an alloy such as CoCrPt, CoCrTa, CoCrPtTa and CoCrPtB, targets with Cr concentrations of 18 to 26At % are used. A larger portion of the Cr stays in the grain boundaries, but still a considerable portion of the Cr remains within the grain. Further Pt of 8 to 14At % is added to obtain the necessary coercivity Hc and magnetic anisotropy Ku. The Co within the grain is thus diluted with the Cr and other additives, thereby considerably reducing the saturation magnetization Ms of the magnetic layer 3. However, for the magnetic recording medium which has the magnetic layer 3 made of a single-layer structure, such dilution of the Co is inevitable considering the segregation which is needed to obtain a high SNR.
FIG. 3 is a diagram showing a trend in increasing SNR with increasing coercivity. In FIG. 3, the ordinate indicates the SNR (dB) of the magnetic recording medium (or media SNR), and the abscissa indicates the coercivity Hc (Oe) of the recording medium (or media coercivity). The media coercivity can be increased by forming the magnetic layer 3 from a magnetic material having a high magnetic anisotropy Ku. As outlined above, the high magnetic anisotropy Ku of the magnetic material should be accompanied by an increase in the saturation magnetization Ms to have a good overwrite performance.
For a given composition, the coercivity Hc can be improved by improving the in-plane c-axis orientation of the magnetic layer 3. The improved in-plane c-axis orientation also results in an increase in remanent magnetization Mr which decreases dc-erased noise. The in-plane c-axis orientation is promoted for the magnetic layer 3 made of Co-based alloys having preferred orientations of the (11{overscore (2)}0) face grown epitaxially on the (200) face of the Cr underlayer 2 or, the (10{overscore (1)}0) face grown epitaxially on the (211) face of a NiAl underlayer 2. A lattice mismatch between the underlayer 2 and the magnetic layer 3 leads to stacking faults which may decrease the coercivity Hc. The lattice mismatch may be minimized by alloying the Cr underlayer 2 with an element such as Mo, V and W.
A few monolayers in the magnetic layer 3 may be non-magnetic due to stresses or defects in the hcp-bcc interface. In such a case, non-magnetic or slightly magnetic hcp thin intermediate layer may be used to decrease such defects and to improve the in-plane coercivity, as proposed in U.S. Pat. Nos. 5,820,963 and No. 5,848,386, for example.
A decrease in the media noise may also be achieved by decreasing a parameter Mrt, where Mr denotes the remanent magnetization and t the thickness of the magnetic layer 3. The parameter Mrt can be decreased by reducing the media thickness or by decreasing the saturation magnetization Ms of the magnetic layer 3. However, the decrease in the media thickness beyond a certain point does not decrease the media noise any further. The magnetic grains become thermally unstable, thereby decreasing the coercivity Hc and thus increasing the media noise. Thus, for a particular composition of the magnetic layer 3, there is an optimum media thickness where the SNR is a maximum. Therefore, for a particular Mrt value, it is better to have a higher saturation magnetization Ms and a smaller media thickness, than to have a lower saturation magnetization Ms and a larger media thickness combination.
As described above, there is a need to decrease the media noise in order to realize a high-density recording performance for the magnetic recording medium. There is also a need to improve the overwrite performance and to improve the thermal stability of the magnetic recording medium. In order to obtain a good overwrite performance without adversely affecting the thermal stability, however, there is a need to increase the saturation magnetization Ms of the magnetic layer. But for a magnetic recording medium having the magnetic layer with the single-layer structure, increasing the saturation magnetization Ms usually affects the intergranular interaction, thereby deteriorating the SNR performance.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium and magnetic storage apparatus, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a magnetic recording medium and a magnetic storage apparatus, which can simultaneously improve the SNR, overwrite performance and thermal stability.
Still another object of the present invention is to provide a magnetic recording medium provided with a substrate and a magnetic layer disposed above the substrate, wherein the magnetic layer includes an initial layer located closer to the substrate and a final layer located further away from the substrate than the initial layer, a saturation magnetization MSa of the initial layer, a saturation magnetization MSb of the final layer, and a saturation magnetization MStot of the magnetic layer as a whole satisfy a relationship MSa less than MStot less than MSb. According to the magnetic recording medium of the present invention, it is possible to simultaneously improve the SNR, overwrite performance and thermal stability.
A further object of the present invention is to provide a magnetic recording medium provided with a substrate and a magnetic layer disposed above the substrate, wherein the magnetic layer includes an initial layer located closer to the substrate and a final layer located further away from the substrate than the initial layer, a saturation magnetization MSa of the initial layer, a saturation magnetization MSb of the final layer, and a saturation magnetization MStot of the magnetic layer as a whole satisfy a relationship MSa greater than MStot greater than MSb According to the magnetic recording medium of the present invention, it is possible to simultaneously improve the SNR, overwrite performance and thermal stability.
In the magnetic recording medium, the magnetic layer as a whole may have a signal-to-noise ratio higher than that obtained solely by the initial layer and also higher than that obtained solely by the final layer.
In addition in the magnetic recording medium, the magnetic layer as a whole may have a coercivity higher than a coercivity of a medium solely made up of an equally thick initial layer and lower than a coercivity of a medium solely made up of an equally thick final layer.
In the magnetic recording medium, the initial layer may be made of a CoCrx1 alloy where x1 denotes an At % content satisfying 15 less than x1 less than 27, and the final layer may be made of a CoCrx2 alloy where x2 denotes an At % content satisfying 10 less than x2 less than 24.
Further, in the magnetic recording medium, the initial layer may be made of a CoCrBy alloy where y denotes an At % content satisfying 1 less than y less than 15, and the final layer may be made of a CoCrBz alloy where z denotes an At % content satisfying 0xe2x89xa6z less than 10.
A further object of the present invention is to provide a magnetic storage apparatus comprising a head, and at least one magnetic recording medium having a magnetic layer structure recited above. According to the magnetic storage apparatus of the present invention, it is possible to simultaneously improve the SNR, overwrite performance and thermal stability.